There is a great need to increase the fuel efficiency of internal combustion engines (ICEs). In the past, a conventional thermal barrier coating (TBC) usually prepared by thermal spray technology was used in an attempt to increase ICE fuel efficiency through the reduction of heat transfer loss. However, while TBC increased the mechanical durability of coated components at high combustion temperatures, it did not obviously improve the fuel efficiency of ICEs This is because TBC can retain constantly high surface temperatures, likely causing a pre-ignition or knocking problem. In order to avoid these problems, the fuel ignition time must be retarded, resulting in lower combustion efficiency.
It is also difficult for thermal spray coating technology to be deposited on small channels and irregular internal surfaces. Enamel and sol-gel coating technologies can be used to deposit TBC, but a series of post-heat treatments may be required. A hard anodizing process can also create TBC, but the resulting oxide coating has an amorphous structure with high internal residual tensile stresses causing many surface cracks; these cracks may lead the coating to have a peeling problem under cyclic heating and cooling ICE conditions. Furthermore, it is difficult for a hard anodizing process to make a coating thicker than 70-80 microns.
Plasma electrolytic oxidation (PEO) or micro-arc oxidation technology [Nie and Matthews et al., Surface & Coatings Technology, 1999] can produce an oxide coating as well. However, the current trend in oxidation processes is to make the coating dense with a limited porosity, and it is challenging to coat a local surface area of an ICE component since a complicated masking technique is needed on surfaces where no coating is required.
Most importantly, a coating prepared by use of the conventional thermal spray, anodizing process, PEO or micro-arc oxidation coating process, does not have the flexibility to meet the different thermal management requirements of the various ICE components. For a cylinder bore, a high thermal conductive coating surface is desired in order to cool air within the cylinder during the intake stroke; for a cylinder head, a low thermal conductive and diffusive coating surface is beneficial to reduce heat loss; and for a piston, a piston crown surface with low thermal diffusivity and low thermal capacity would be needed to decrease heat rejection loss and swiftly adapt to temperature oscillation between combustion and intake strokes. The coatings currently in use have not been and may not be able to meet those different thermal property requirements at the same time.
The invention hereby relates to an innovative plasma oxidation method for making an air-containing oxide coating which provides the thermal properties needed for the different functions of ICE components. The said coating method is to tailor the volume percentage of air pockets in the oxide coating, thus achieving an optimized thermal resistance, thermal inertia and heat transferring property for various engine components.